Connecting devices of this type are primarily used in machine tools having an automatic tool exchange in particular for machining centers with a center tool-feeding apparatus. This tool-feeding apparatus, which is primarily provided with a steep-angle taper to receive the tool, operates in most machining centers through a cup-spring package with hydraulic relaxation. A head bolt arranged on the steep-angle taper is pulled into the spindle by a tong-like gripping means clamping the steep-angle taper. The clamping path needed in the case of the steep-angle taper amounts to approximately 6 to 10 mm depending on the size of the spindle. In order to achieve at a given spindle size an increased support compared with the steep-angle taper connection, it has already been suggested to replace the steep-angle taper receiving means with a cylindrical receiving means having a front-face planar surface support for the tool (Magazine "Werkstatt und Betrieb" 119 (1986), Pages 797 to 801) and to use at the same time the center tool-feeding apparatus already existing in many machine tools as a clamping mechanism.
Thus, the conventional tool of the type mentioned above has two clamping bolts radially movably supported in the fitting pin, which clamping bolts can be moved diametrically outwardly by a clamping member designed as a drawing part and axially movable by the clamping mechanism arranged on the spindle. In their outwardly moved clamping position, the clamping bolts, with their truncated-cone-shaped tips, extend into a conical recess in the area of the bore wall of the receiving means in order to pull an annular planar surface adjoining the fitting pin against a planar surface adjoining the fitting aperture and to brace same. The clamping bolts were in the release position pulled so far back into the fitting pin that the fitting pin can be pulled out of the fitting aperture during the tool exchange. The clamping force transferable onto the tool coupling at a given feeding force provided by the center clamping mechanism is larger the smaller is the wedge angle of the wedge drive formed by the tapered surfaces of the clamping member and of the clamping bolt. On the other hand, it is not possible to reduce at random the size of the wedge angle of the known tool coupling since this effects also the radial stroke of the clamping bolts as a given axial adjustment path of the clamping mechanism becomes smaller. This disadvantage exists in particular in the case of larger tools, which besides an increased clamping force require also a greater adjusting stroke of the clamping bolt. Thus, the known clamping mechanisms with cup-spring packages cannot easily be utilized mainly in larger tools with planar-surface bracing. Instead, hydraulic tool-clamping means have already been utilized, which enable a larger adjustment path. The here needed, complicated change-over of machine tools and machining centers limits, however, the area of use of the tools with planar-surface bracing.